In the continuously variable transmission using a circular belt member (hereinafter, referred to as a belt) such as a belt and a chain, a hydraulic pressure is supplied to a primary pulley and a secondary pulley. A clamping force is added to the belt by this hydraulic pressure. A torque is transmitted by using a frictional force generated by the clamping force. In this continuously variable transmission, the torque is inputted from the engine. The torque is also inputted from the driving wheels in accordance with the state of a road surface. For example, the torque inputted from this driving wheel is increased at the traveling on the rough road such as a dirt. Accordingly, the frictional force is deficient with respect to the transmission torque (transmitting torque). The slippage between the pulley and the belt may be generated.
As a countermeasure for the above problem, a patent document 1 proposes an art in which when it is judged that the road surface state is a rough road, the clamping force is increased relative to a case where it is judged that the road surface state is a smooth road, so as to prevent a slippage of the belt. In particular, the hydraulic pressure of the hydraulic fluid supplied to a hydraulic pressure actuator of the driven pulley (the secondary pulley) is adjusted so as to obtain the desired clamping force. Moreover, the hydraulic pressure supplied to a hydraulic pressure actuator of the driving pulley (the primary pulley) is adjusted so as to attain the desired transmission gear ratio.
However, for increasing the clamping force of the secondary pulley when it is judged that the road surface state is the rough road like the art of the patent document 1, it is necessary to increase the hydraulic pressure of the hydraulic fluid supplied to the hydraulic actuator of the secondary pulley.
Moreover, in case of a vehicle (automobile), in general, the hydraulic pressure is produced by a mechanical oil pump driven by an engine for the traveling. The hydraulic fluid discharged from the oil pump driven by the engine is used as the hydraulic fluid supplied to the hydraulic pressure actuator of the pulleys of the continuously variable transmission for the vehicle.